One known family of porous crystalline materials are zeolitic materials, which are based on the 3-dimensional, four-connected framework structure defined by corner-sharing [TO4] tetrahedra, where T is any tetrahedrally coordinated cation. Among the known materials in this family are silicates that contain a three-dimensional microporous crystal framework structure of [SiO4] corner sharing tetrahedral units, aluminosilicates that contain a three-dimensional microporous crystal framework structure of [SiO4] and [AlO4] corner sharing tetrahedral units, aluminophosphates that contain a three-dimensional microporous crystal framework structure of [AlO4] and [PO4] corner sharing tetrahedral units, and silicoaluminophosphates (SAPOs), in which the framework structure is composed of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units. Included in the zeolitic family of materials are over 200 different porous framework types, many of which have great commercial value as catalysts and adsorbents.
Zeolitic imidazolate frameworks or ZIFs have properties similar to inorganic zeolitic materials. ZIFs are based on [M(IM)4] tetrahedral bonds in which IM is an imidazolate type linking moiety and M is a transition metal. These materials are generally referred to as zeolitic imidazolate frameworks or ZIFs since the angle formed by imidazolates (IMs) when bridging transition metals is similar to the 145° angle of the Si—O—Si bond in zeolites. ZIF counterparts of a large number of known zeolitic structures have been produced. In addition, porous framework types, hitherto unknown to zeolites, have also been produced. Discussion of this research can be found in, for example, the following publications from Yaghi and his co-workers: “Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks”, Proceedings of the National Academy of Sciences of U.S.A., Vol. 103, 2006, pp. 10186-91, “Zeolite A Imidazolate Frameworks”, Nature Materials, Vol. 6, 2007, pp. 501-6, “High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture”, Science, Vol. 319, 2008, pp. 939-43, “Colossal Cages in Zeolitic Imidazolate Frameworks as Selective Carbon Dioxide Reservoirs”, Nature, Vol. 453, 2008, pp. 207-12, “Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate Frameworks and their Carbon Dioxide Selective Capture Properties”. Journal of the American Chemical Society, Vol. 131, 2009, pp. 3875-7, “A Combined Experimental-Computational Investigation of Carbon Dioxide Capture in a Series of Isoreticular Zeolitic Imidazolate Frameworks”, Journal of the American Chemical Society, Vol. 132, 2010, pp. 11006-8, and “Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks”, Accounts of Chemical Research, Vol. 43, 2010, pp. 58-67.
Much of this work on ZIF structures is summarized in U.S. Patent Application Publication No. 2007/0202038, the entire contents of which are incorporated herein by reference. In particular, the '038 publication discloses a zeolitic framework, comprising the general structure: M-L-M, wherein M comprises a transition metal and L is a linking moiety comprising a structure selected from the group consisting of I, II, III, or any combination thereof:
wherein A1, A2, A3, A4, A5, A6, and A7 can be either C or N, wherein R5-R8 are present when A1 and A4 comprise C, wherein R1, R4 or R9 comprise a non-sterically hindering group that does not interfere with M, wherein R2, R3, R5, R6, R7, R8, R10, R11, and R12 are each individually an alkyl, halo-, cyano-, nitro-, wherein M1, M2, M3, M4, M5, and M6 each comprise a transition metal, wherein when the linking moiety comprises structure III, R10, R11, and R12 are each individually electron withdrawing groups.
In a more recent work by Ni et al., the structure and synthesis of mixed-valence ZIFs are disclosed in U.S. Patent Application Publication No. 2010/0307336. Specifically, the authors disclose in the '336 publication a porous crystalline material having a tetrahedral framework comprising a general structure. M1-IM-M2, wherein M1 comprises a metal having a first valency, wherein M2 comprises a metal having a second valency different from said first valency, and wherein IM is imidazolate or a substituted imidazolate linking moiety. Such materials can sometimes be described as iso-structural to known ZIF materials.
ZIF materials may be conventionally prepared by dissolving sources of metal ions and sources of imidazolate or substituted imidazolate linkers in an appropriate solvent to form a reaction mixture and then maintaining this reaction mixture under conditions sufficient to form the crystalline ZIF materials as a precipitate. For example, in U.S. Patent Applicaton Publication No. 2007/0202038, it is stated that ZIF materials may be prepared using solvothermal techniques. These techniques may involve combining a hydrated metal salt (e.g., nitrate) and an imidazole-type organic compound in an amide solvent, such as N,N-diethylformamide (DEF), followed by heating (e.g., to 85-150° C.) the resultant solutions for 48-96 hours to precipitate with a zeolitic framework.
One problem with the precipitation or solvothermal method for forming ZIF materials is that it affords one little or no control over the framework type of the zeolitic material obtained. For example, as reported in the literature, when a ZIF is crystallized from a solution of zinc ions and 5-azabenzimidazole molecules, the resulting ZIF material (i.e., ZIF-22) tends to have the LTA framework type; see, for example, the aforementioned article “Zeolite A Imidazolate Frameworks”, Nature Materials, Vol. 6, 2007, pp. 501-6 by Yaghi and his co-workers.
Another problem with the precipitation or solvothermal method for forming ZIF materials is that it may be difficult or impossible to incorporate a desired functional group on an imidazolate-type linker into a ZIF of the desired framework type. As noted above, conventional synthesis of ZIF-22 results in a LTA structure having a 5-azabenzimidazolate linker. The 5-aza group on the linker has functionality as a Lewis base, so it could have affinity for a gas molecule with an electrophilic center, such as carbon dioxide. However, ZIF-22 is not exceptional among ZIF materials in terms of CO2 adsorption; see Example 5 of the present application and see also the CO2 adsorption data, reported in the aforementioned Nature Materials 2007 article, for ZIF-20, which is the purine counterpart of ZIF-22 (i.e., having “aza” functional groups at both the 5- and 7-positions instead of only at the 5-position. In fact, neither ZIF-22 nor ZIF-20 was even mentioned by Yaghi and his co-workers when they reviewed the CO2 adsorption performance of ZIFs in “Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks”, Accounts of Chemical Research, Vol. 43, 2010, pp. 58-67. Without being bound by theory. ZTF-22's mediocre performance in CO2 adsorption is believed to be a result of the 5-azabenzimidazolate linkers within the structure not being close enough to each other, which in turn can be due to the presence of large cages in the framework type LTA (i.e. small β cages separated by large α cages). Therefore, in order to enhance CO2 adsorption, it would be extremely desirable to have a ZIF composition that has the 5-azabenzimidazolate linker and a reduction in or absence of large cages, e.g. the framework type SOD (i.e., only small β cages), which has not been produced by the conventional method despite intense study on this synthesis system.
Accordingly, it would be desirable to provide methods for preparing ZIF materials affording greater control over the resulting structure, for example, affording the possibility of incorporating desired functional groups into ZIF materials having a desired framework type.